A method for filtrating a liquid with a separation membrane is a method wherein the liquid which is a liquid to-be-filtrated is held in touch with the separation membrane, and a filtrated liquid is obtained by applying a pressure from the side of the liquid to-be-filtrated or by holding a penetration side at a negative pressure. On this occasion, a membranous filtration resistance increases for such a reason that any nonmembranous penetrative substance contained in the liquid to-be-filtrated attaches onto a membrane surface, that the nonmembranous penetrative substance accumulates into membrane pores, or that the nonmembranous penetrative substance having deposited is consolidated by a pressure acting on this nonmembranous penetrative substance. When the membranous filtration resistance has enlarged, a membranous filtration flowrate decreases in a case where the membranous filtration is performed by applying a constant membranous filtration pressure, and a transmembrane pressure increases in a case where the membranous filtration is performed by holding the membranous filtration flowrate constant. In the former case, a planned flowrate cannot be secured, while in the latter case, energy for increasing the pressure is required, and simultaneously, a burden for the separation membrane increases. Such an increase behavior of the membranous filtration resistance is greatly influenced by a membranous filtration condition, and it is an important factor for continuously performing the membranous filtration. In other words, if the temporal variation of the membranous filtration resistance, the membranous filtration flowrate or the transmembrane pressure can be exactly predicted, it is permitted to find out the membranous filtration condition for performing an efficient and stable filtration.
Besides, as a method for suppressing such an increase of the membranous filtration resistance, there is a method wherein the membranous filtration is performed while the membrane surface is being washed, such as one wherein the membrane surface is exposed to air by an air sprinkler which is disposed under the membrane, one wherein the liquid to-be-filtrated is circulated in a direction parallel to the membrane surface, by utilizing a pump, or one wherein the separation membrane is moved or vibrated. In such a membranous filtration method, the increase of the membranous filtration resistance can be suppressed by sufficiently performing the membrane wash, but energy is required for such a wash of the membrane surface, and it increases a running cost to perform the membrane wash more than is necessary. That is, it is important for the running rationalization of a membranous filtration apparatus to perform the membrane surface wash necessarily and sufficiently.
Besides, also a membrane surface wash with chemicals or the like is sometimes performed after temporarily suspending the filtration. In this case, a filtration running efficiency lowers due to the temporary suspension of running, and a chemicals cost is also required, so that a timing at which the membrane wash is performed is important.
Therefore, the exact prediction of the temporal variation of the membranous filtration resistance during the filtration is very effective because a quantitative membranous filtration condition can be determined. It is permitted to determine, for example, the minimum exposure airflow which is necessary for suppressing the increase of the membranous filtration resistance.
A mathematical formula model for predicting the variation of the membranous filtration resistance during such a membranous filtration is proposed in Patent Document 1. This method predicts the membranous filtration resistance in such a way that the increase or decrease of a nonmembranous penetrative substance which is contained in the liquid to-be-filtrated attaching on the surface of the separation membrane is expressed as the mathematical formula model. Besides, a mathematical formula model for predicting the membranous filtration resistance is stated also in Non-patent Document 1. With this method, the increase or decrease of an extracellular polymeric substance amount (hereinbelow, termed the “EPS”) attaching on the filtration membrane is expressed as the mathematical formula model, and the membranous filtration resistance is predicted on the basis of the EPS.
Patent Document 1: JP-A-2007-14948
Non-patent Document 1: H. Nagaoka, “MODELING OF BIOFOULING BY EXTRACELLULAR POLYMERS IN A MEMBRANE SEPARATION ACTIVATED SLUDGE SYSTEM”, Wat. Sci. Tech. Vol. 38, No. 4-5, pp 497-504, 1998.